Current spectrum allocation models result in under-utilization of licensed (primary) spectrum over space and time, as is the case e.g. in VHF-UHF bands. This has led to a growing interest in flexible spectrum usage models. One such model provides a secondary usage of a spectrum that has been licensed for a primary wireless system. The secondary usage is enabled by a secondary cognitive wireless system. The operation of the secondary cognitive wireless system should not cause a harmful interference to primary system users. This requirement can be met if the secondary cognitive wireless system operates on portions of the spectrum that are not used for primary user transmissions. To determine these portions, the spectrum is monitored for primary user activity. This is done by spectrum sensing.
An energy detection may be used to determine by means of a single sensing unit whether or not a signal of known bandwidth is present in a given spectrum region of interest. More advanced detection techniques that exploit signal features and higher-order statistical properties have also been considered. In a cognitive wireless system, a number of such sensing units are available, and a detection performance can be improved by combining information from multiple sensing units.
Two approaches for combining information from multiple sensing units have been proposed up to now. On the one hand, each sensing unit can make a hard detection decision and transmit a binary result (1/0) thereof to a central unit, where the received bits are combined. A commonly used rule for combining the bits is a majority rule, where a final decision of signal present is made if a majority of received bits are 1s. The advantage of this approach is that the amount of signaling is low. However, making a hard detection decision results in a loss of implicit signal information, which cannot be exploited further at the central unit. This in turn impacts the final detection performance.
On the other hand, each sensing unit may simply transmit raw observations or some other form of the observations to a central unit. While this approach can result in a better detection performance, the amount of signaling involved therewith is unacceptably large.
A. Ghasemi and E. S. Sousa, “Impact of user collaboration on the performance of sensing-based opportunistic spectrum access”, IEEE VTC Fall, pp. 1-6, September 2006, discloses that either energy values or 1-bit hard decisions are transmitted from individual sensors to a central fusion unit and are combined there to effect a spectrum sensing.